Telephone-exchange system.



C. W. K'ECKLEB. Y

TELEPHONE EXCHANGE SYSTEM.

APPLICATION FILED APR. 24. 1917.

1,257,362. Patented Feb. 26,1918.

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I l ii Mremor: Char/es W/fec/r/e C. W. KECKLER.

TELEPHONE EXCHANGE SYSTEM.

APPLICATION FILED APR. 24. 1917.

Patented Feb.26,1 918.

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//7 yen/or Char/es W/Ie ck/er c. w. KECKL-ERJ TELEPHONE EXCHANGE SYSTEM. APPLICAITION FILED APR.24, 1917.

Patented Feb 26', 1918.

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srn ns CHARLES VJ. KECKLER, OF NEWARK, NEW JERSEY, ASSIGNOR T9 VTESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW/V YORK, N. Y., A CORPORATION OF NEW YORK;

TELEPHONE-EXCHANGE SYSTEM.

Application filed April 24, 1917.

To all whom 2'25 may concern:

Be it known that 1, CHARLES W. Knonnnn, a citizen of the United States, residing at Newark, in the county of Essex and State of New Jersey, have invented certain new anduseful Improvements in Telephone-EX- change Systems, of which the following is a full, clear, concise, and exact description.

The invention relates to telephone exchange systems, and more particularly to such systems of the central energy type, its object being to provide improved signaling means for such systems.

In accordance with a feature of this invention, a link circuit having a signaling device and a supervisory relay is provider with a difierentially-wound local circuit relay which cooperates with the supervisory relay to cause the intermittent operation of the signaling device, whereby said device furnishes a characteristic signal.

This invention is illustrated in the accompanying drawings; wherein,

Figure 1 shows two ordinary telephone line circuits and a link circuit embodying the present invention;

Fig. 2 shows a link circuit embodying a modification of this invention in which an answering supervisory signaling device is intermittently operated to furnish a recall signal from a connected calling line and a calling supervisory signaling device is intermittently operated to furnish a disconnect signal from either of the connected lines,

Fig. 3 shows two ordinary line circuits and a link circuit embodying another modification of this invention, in which an answering supervisory signaling device is intermittently operated to furnish a recall signal from a connected calling line, and the calling supervisory signaling device is operated continuously to furnish a disconnect signal from either of the connected lines;

Fig. 4: shows a link circuit embodying this invention in which the apparatus is shown in one of its more simple forms;

Fig. 5 shows two ordinary telephone line circuits and a link circuit embodying a still further modification of this invention in which a calling supervisory signaling device is intermittently operated to furnish a recall Specification of Letters Patent.

Patented Feb. 26, 1918.

Serial No. 164,171.

signal from the original calling station connected with the link circuit;

Fig. 6 shows a link circuit adapted for operation with the line circuits of Figs. 1, 3 or 5, and in which the line signaling device of a connected line is intermittently operated to furnish a recall signal; and

Fig. 7 shows a link circuit embodying an additional modification of this invention, in which both the answering and calling supervisory signaling devices are intermittently operated to furnish a recall signal from a connected calling line.

In the various figures of the drawings, similar pieces of apparatus will be designated by the same reference character. It is believed that this invention is not of such a character as to require a detailed description of the apparatus appearing in each of the figures and for this reason only a description of the operation will be included.

Figure 1.

Subscriber A having signaled the central oliice in the usual manner, the operator inserts answering plug 15 into line jack 16, whereupon dilierentially-wound relay 17 and cut-oil relay 18 are opera ted over a circuit from battery through the upper winding of relay 1T, sleeve contacts of plug 15 and jack 16 and the winding of cut-oil relay 18 to ground. The energization of the upper winding of relay 17 is suilicient to attract the two upper and the three lower armatures. Answering supervisory relay 19, upon the insertion of the plug in the jack, is also operated. The actuation of relay 17 connects the operators telephone set to the cord circuit, and the operator, upon being advised that connection is desired with the line of subscriber l3, inserts calling plug 25 into line jack where-upon relay '27 and cut-oil relay 28 are operated. The operation of relay 2'? causes calling supervisory lamp 99 to be lighted, and completes a circuit for the lower winding of relay 17, which may be traced from battery through the lower Wind-- ing of relay 17, left-hand alternate contact of relay 27, and contact 20 of relay 17 to ground. Since the direction of flow of current through the two windings of relay 17 is opposite, this relay releases its armatures,

In releasing, the normal contacts of the upper armatures of this relay 17 are made before the alternate contacts of the lower armature are broken. Therefore, although contact 20 is opened when relay 17 releases, the circuit including the lower winding thereof is maintained through the normal contact 21 of relay 17, alternate contact of supervisory relay 19, and contact 22 of relay 17 to ground. The release of the lower armatures of relay 17 also disconnects the operators telephone set from the line. Subscriber B may be signaled in any desired manner, and in response to such a signal, removes the receiver from the switchhook, causing the actuation of calling supervisory relay 30, whereupon the calling supervisory lamp 29 is etiaced.

Upon the conclusion of the conversation, both subscribers restore the receivers to the switchhooks, causing the release of supervisory relays 19 and 30, the release of the latter causing the lighting of calling supervisory lamp 29. The release of relay 19, when subscriber A restores his receiver to the switchhoolr, opens the hereinbefore described circuit through the lower winding of relay 17.

Another circuit, however, exists for this lower winding which includes left-hand contact of relay 27 and resistance 31. This resistance 31 serves to unbalance the inductive effect of the two windings of relay 17, whereupon the relay is sufliciently energized to cause the attraction of its upper armatures, the lower armatures remaining in their normal position. The closure of contact 23 of relay 17 completes the circuit for answering supervisory lamp 32, which is now lighted to furnish a disconnect signal.

Should the calling subscriber A desire to initiate another call before the plugs of the cord circuit C are removed from the lines A and B, the removal of the receiver by subscriber A will cause the operation of answering supervisory relay 19, whereupon lamp 32 will be intermittently operated to furnish a recall signal. The circuit over which the lamp is intermittently lighted may be traced from battery through the lamp 32, contact 23 of relay 17, alternate contact of supervisory relay 19, contact 24 of relay 17, contact 33 of relay 27, and interrupter 34 to ground. The operator, observing the flashing of the answering supervisory lamp 32, removes the calling plug 25 from jack 26, whereupon relay 27 releases to etace the calling supervisory lamp 29, and to open the flashing circuit of lamp 32. In addition, the circuit through the lower winding of relay 17 is interrupted at the left-hand contact of relay 27 whereupon relay 17 is sutiiciently energized to attract all of its armatures, thereby connecting the operatcrstelephone set to the cord circuit. The operator may now converse with subscriber A, and complete 1n the manner hereinbefore described any connection desired.

Figure 2.

Assuming that the operator, in answering a call from subscriber A, employs the cord circuit of Fig. 2, differentially wound relay 37 thereof and cut-0E relay 18 are operated ever a circuit from battery through the lower winding of relay 37, sleeve contacts of plug 15 and ack 16, and the winding or cut ofl relay 18 to ground. Relay 37, in operating, attracts all of its armaturcs. Upon the insertion of the plug into the jack of a calling line, answering supervisory relay 19 also operates. Relays 37 and 19 being operated, answering supervisory lamp 32 is intermittently operated over a circuit from battery through the alternate contact of relay 19, contact 38 of relay 37, lamp 32 and interrupter 3st to ground. The operator may now actuate listening key 39 to connect the operators telephone set to the cord circuit. Upon the operation of this listening key, a circuit through the upper winding of relay 37 is established over a path from battery through contact $0 of listening key 3'9, and upper winding of relay 37 to ground. Since relay 37 is diiierentially wound, it now releases its armatures, whereupon a second circuit through the upper winding is established from battery through the alternate contact of 19, contact 11 of relay 37 and the upper winding thereof to ground. Relay 37, in releasing its armature, also stops the intermittent operation of answering supervisory lamp 32.

After conversing with the calling sub scriber, the operator may restore the listening key to its normal position and complete the connection with the line of desired aarty, for example, subscriber B,by the insertion of calling plug 25 into the line jack 26 of the desired lino. Relay l2 of the cord circuit and the cut-off relay of the line are thereupon operated. Relay 42 in operating causes the lighting of calling supervisory lamp +15 over a circuit from battery through contact 4: 1 of relay 37, loft-hand contact of relay 4:2, lamp 45, contact 46 of relay 37, normal contact of calling supervisory relay 30, left-hand normal contact oi relay 13, and middle contact of relay 12 to ground.

Subscriber B may be signaled in any de sired manner. in answering, the removal of the receiver from the switchhook causes the operation of callin supervisory relay 30, whereupon relay 43 is actuated over a circuit from battery through contact A- of relay 37, lett-hand contact of relay l2, calling supervisory lamp as, contact 16 of relay 37 alternate contact of calling supervisory relay 30, and right-hand normal contact of relay 43 and the winding thereof to ground.

Relay 4:3, in operating, establishes a locking circuit for itself from battery through the right-hand contact of relay 42, right-hand alternate contact of relay 43 and the wind ing thereof to ground. The operation of calling supervisory relay 30 also extinguishes the calling supervisory lamp 15.

Upon the conclusion of the conversation, the restoration of the receivers to the switchhooks causes the release of supervisory relays 19 and 30. The release of relay 19 opens the hereinbefore traced circuit for the upper winding of relay 37, which now operates, establishing a circuit over which lamp is flashed. This circuit may be traced from battery through normal contact of supervisory relay 19, lamp 15, contact d7 of relay 37 and interrupter 3% to ground. This circuit is under control of the calling subscriber. Should subscriber B have restored the receiver to the switchhook before sub scriber A, lamp i5 would be flashed to indicate a disconnect signal, the circuit therefor being from battery through contact 44 of relay 37, left-hand contact of relay 4E2, lamp 15, contact :6 of relay 37, normal contact of relay 30, left-hand alternate contact of relay %3 and interrupter 34 to ground.

After both receivers have been restored to the switchhooks and before the plugs of the cord circuit are withdrawn from the jacks of the two lines, should subscriber A desire to make a recall, the removal of the receiver from the switchhook by subscriber A would cause the answering supervisory lamp 32 to be flashed to furnish a recall signal to the operator Upon the removal of the receiver from the switchhook by subscriber A, answering supervisory relay 19 is operated, whereupon a flashing circuit for lamp 32 is completed from battery through alternate contact of answering supervisory relay 19, contact 38 of relay 37, lamp 32 and interrupter to ground. The actuation of the listening key 39 by the operator in answer to the flashing of lamp 32 completes the circuit through the upper winding of relay 37, whereupon this relay eleases its armatures and establishes the hereinbefore traced second circuit through the upper winding thereof. The flashing of lamp 32 is thereupon stopped. The operator, upon being advised of the number desired, may withdraw the plug from the jack and insert it in the jack of a desired line. Then the plug is withdrawn from the original jack, relays and T3 are released, and when'the plug is inserted in the. jack of the desired line, relay again operates.

Figure 3.

The insertion of calling plug 15 of the cord circuit G into jack 16, in response to a call by subscriber A, causes the operation of diflerentially Wound relay 57. Answering supervisory relay 19 also operates, thereby causing the flashing of a recall lamp 32 over a circuit from battery through the alternate contact of relay 19, upper alternate contact of relay 57, lamp 32, and interrupter 3a to ground. The. actuation of listening key 39 connects the operators telephone set to the cord circuit, and also completes a circuit through the upper winding of clifferential relay 57, whereupon this relay releases its armat-ures, and establishes another circuit through its upper winding from battery through the alternate contact of relay 19, upper normal contact of relay 57 and the upper winding thereof, to ground. Find-- ing that the line desired is that of subscriber B, the operator restores the listening key 39 to normal position and inserts the calling plug 25 into line jack 26, whereupon cut-elf relay 28 is actuated. Subscriber B may be signaled in any desired manner, and in responding, causes the operation of calling su pervisory relay 30. The operation of relay opens the circuit of the upper winding of differential relay 5S, whereupon this relay ope ates over a circuit including its lower winding.

At the conclusion of the conversation, subscribers A and B restore the receivers to the switchhooks, causing the release of relays 19 and 30. The release of relay 30 causes the lighting of disconnect lamp 59 over a circuit from battery through contact of relay 30, alternate contact of relay 58 and the lamp 59 to ground. The release of relay 19 opens the circuit through the upper winding of relay 57, whereupon this relay operates, closing another circuit through disconnect lamp 59, which may be traced from battery through normal contact of relay 19, lower alternate contact of relay 57 and the lamp 59 to ground.

Before the plugs of the cord circuit C are withdrawn from the jacks of the two lines, should subscriber A desire to initiate another call, the removal of the receiver from the switchhook causes the operation of relay 19, whereupon the recall lamp 32 is intermittently operated over a circuit from battery through the alternate contact of relay 19, upper alternate contact of relay 57, lamp 32 and interrupter to ground. The intermittent operation of this lamp indicates a recall signal to the operator, who may now actuate listening key 39 to connect the operators telephone set across the cord circuit, and to cause the release of relay, 57 in the manner hereinbefore described, which stops the intermittent operation of recall lamp 32, The removal of the plug 25 from the jack 26 by the operator, causes the release of differential relay 58.

F igure 4.

Assuming that the operator in answering kit the call from subscriber A, employs the cord circuit of Fig. 4, the insertion of the plug 15 thereof into jack 16 causes the operation of cut-oil relay 18 in the manner hereinbefore described, and also causes the operation of a dinerentially-wound relay 67 over a circuit which includes its upper winding. Answering supervisory relay 19 also operates, whereupon answering supervisory lamp 32 is intermittently operated over a circuit from battery through lamp 32, upper alternate contact of differential relay 67, alternate contact of relay 19, lower alternate contact of relay (57 and the interrupter 34 to ground. The operator may actuate listening key 39 to connect toe operators tele phone set to the cord circuit, and in so doing, completes a circuit through the lower winding of relay 67 which includes the contact a0 of listening key Relay 67 thereupon releases its armatures, stopping the intermittent operation of lamp 32 and establishing another circuit through its lower *inding which may be traced from battery through the lower winding and lower normal contact of relay 67, alternate contact of relay 19 and upper normal contact of relay 67 to ground. The connection with a called line may be completed in the usual manner, after listening key 39 has been restored to normal position. Upon the restoration of the receiver to the switchhook by subscriber A at the conclusion of the conversation, relay 19 releases, opening the hereinbefore traced circuit through the lower winding of relay 67, which now operates, causing the continuous operation of answering supervisory lamp 32 over a circuit from battery through lamp 32, upper alternate contact of relay 67 and normal contact of relay 19 to ground. If, before the operator withdraws the plugs from the jacks, subscriber A desires to initiate another call, the removal of the receiver from the switchhook causes the operation of relay 19, whereupon lamp 32 is intermittently operated over the hereinbefore traced circuit to furnish a recall signal.

Figure 5.

In answering a call from the subscriber on line A, the operator inserts the answering plug of cord circuit G into line jack 16 of line A, whereupon difiercntially-wound relay 77 operates through its upper winding in series with the cut-off relay 18 which also operates. The answering supervisory relay 19 also operates. Upon the operation of the differentially-wound relay 77, relay 7 8 operates over a circuit from battery through the winding of relay 78, lower alternate contact of relay 77, and alternate contact of supervisory relay 19 to ground. Upon the operation of relay 78, the calling supervisory lamp 29 is intermittently operated to furnish a signal. The operator, upon actuating listening key 39, connects the operators telephone set to. the cord circuit, and completes a circuit through the lower winding of the difierentially-wound relay 77, which thereupon releases and establishes a second circuit through the lower winding thereof. This second circuit may be traced from battery through the lower winding of relay 77, lower normal contact of relay 77, and alternate contact of supervisory relay 19 to ground. Upon the release of relay 77, relay 78 is released, stopping the intermittent operation of calling supervisory lamp 29. Upon being advised that the line of subscriber B is desired, the operator restores the listening key to normal position and inserts the calling plug 25 into the line jack 26, whereupon relay cut-off relay 28 are operated. The operation of relay 79 causes the calling supervisory lamp 29 to be lighted over a circuit from battery through the lamp 29, normal contact of relay 78, contact of relay 79 and con tact of relay 30. Subscriber B may now be signaled in any desired manner, and in re sponding, removes the receiver from the switchhook, thereby causing the operation of supervisory relay 30, whereupon the calling supervisory lamp 29 is effaced.

At the conclusion of the conversation, the subscribers restore the receivers to the switchhooks, causing the release of relays 19 and 30. The release of relay 30 causes the lamp 29 to be lighted. Upon the release of answering supervisory relay 19, the circuit.

through the lower winding of relay 77 is opened, and this relay operates, causing answering supervisory lamp 32 to be lighted. These two lamps now serve as disconnect signals.

Should subscriber A desire to initiate another call, before the plugs are withdrawn from the jacks, the removal of tie receiver from the switchhook by subscriber A causes the operation of supervisory relay 19, whereupon relay 78 operates and causes the intermittent operation of a lamp 29, which serves as a recall signal to the operator. In response to this signal, the operator actuates the listening key, which releases relays 77 and 78 in the manner hereinbefore described.

Figure 6.

Assuming that the operator, in answering the call from the subscriber on line A, employs the cord circuit of Fig. 6, differentially-wound relay 87 and answering supervisory relay 19 thereof operate upon the insertion of the answering plug 15 into the line jack 16. The cut-off relay of the line circuit also operates. If the listening key 79 and n 39 is not actuated when relays 87 and 19 operate, relay 88 will be operated over a circuit from battery through the winding of relay 88, lower alternate contact of relay 87 and alternate contact of relay 19 to ground. Relay 88, in operating, connects a coil 89 or low resistance across the tip and ring conductors of the cord circuit, and through the operation of interrupter 3 1, intermittently connects a resistance 86 in parallel with the winding of the cut-oft relay 18. The value of this resistance 86 is such that when it is connected in parallel with the winding of cut-off relay 18, the cut-oii relay will release its'armatures, whereupon line relay 1% will be operated, causing the line lamp 13 to be lighted. This line lamp will be intermittently lighted to furnish a characteristic signal to the operator. Upon the actuation of the listening key 39, the operators telephone set is connected to the cord circuit, and a circuit is established for the lower winding of differential relay 87, whereupon this relay releases, establishing another circuit through the lower winding of this relay which includes the lower normal contact thereof and the alternate contact of answering supervisory relay 19. The release of relay 87 causes the release of relay 88, removing the bridge from across the talking strands of the cord circuit, and removes interrupter 3a and resistance 86 from parallel relation with the winding of the cut-off relay 4 18, thereby stopping the intermittent lighting of line lamp 13. The operator, upon being advised of the number of the called line, restores the listening key to normal position, and completes connection in the usual manner.

At the conclusion of the conversation, subscriber A, in restoring the receiver to the switchhook, causes the release of relay 19, which opens the circuit through the lower winding of relay 87, permitting this relay to operate, whereupon answering supervisory lamp 32 is operated over a circuit from battery through the lamp 32, upper alternate contact of relay 87 and normal contact of relay 19 to ground. The lighting of this lamp serves as a disconnect signal. If, before the plugs are withdrawn from the jacks, subscriber A desires to initiate another call, the removal of the receiver from the switchhook causes the operation of the answering supervisory relay 19, whereupon relay '88 is operated and cut-oft relay 18 is thereafter intermittently released, causing the intermittent lighting of line lamp 13 to furnish a recall signal to the operator. The purpose of resistance 89, which is bridged across the talking strands upon the opera tion of relay 88, is to provide a low resistance path for line relay 14 to insure the operation of this relay each time that the cut-off relay releases. The operator, in response to the recall signal, may now actuate the listening key and complete the connection with any line desired.

Figure 7. Assuming that the operator, in answering a call from subscriber A, employs cord circuit of Fig. 7, the insertion of the answering jack 16, and the winding of cut-off relay 18 to ground. Answering supervisory relay 19 also operates, whereupon, it the listening key 39 is not actuated, answering supervisory lamp 32 will be intermittently oper ated, due to resistance 98 being intermittently connected in parallel with the lamp as a result of the rotation of interrupter 3a. \Vhen the listening key is actuated, the operators telephone set is connected with the cord circuit, and the circuit of the left-hand winding of differential relay 97 is completed, whereupon the relay releases. The circuit including the left-hand winding of relay 97 may be traced from battery through the lefthand winding of relay 97 contact 40 of the listening key 39, contact of supervisory relay 19, right-hand winding of the relay 97, sleeve contacts of the answering plug 15 and line jack 16 and the wincing of cut-ofi relay 18 to ground. Upon the release of relay 97, the left-hand normal contact thereof is closed, thereby establishing a locking circuit including the two windings of relay 97 in series opposing. The left-hand winding of relay 97 is now connected in parallel with answering supervisory lamp 32, thereby efl'acing the lamp.

The operator, upon being advised of the number of the line with which connection is desired, restores the listening key to normal position and inserts the calling plug 25 into the line jack of the desired line, for example, line jack 26, whereupon calling supervisory lamp 29 is lighted and cut-off relay 28 operated. Subscriber 13 may be signaled in the usual manner, and, in responding, removes the receiver from the switchhook and causes the operation of supervisory relay 30. This relay in operating connects a shunt circuit, including the resistance 99, about the lamp 29, thereby effacing the lamp.

At the conclusion or the conversation, subscribers A. and B restore the receivers to the switchhooks, whereupon relays 19 and 30 release. The release of relay 19 opens the circuit which includes the left-hand winding of relay 97, whereupon this relay is actuated and the answering supervisory lamp is lighted to furnish a disconnect signal. 1r, before the plugs removed from the line jacks, subscriber A initiates another call, answering supervisory relay 19 is thereby operated, connecting resistance 98 and interrupter 34: in parallel with lamp 32 and resistance 99 and interrupter 3st in parallel with lamp '29, whereupon these two lamps are intermittently operated to furnish a recall signal to the operator. The operator, in response to this signal, may actuate the listening key, whereupon differential relay 9? releases, stopping the intermittent operation of the two lamps.

In each of the cord circuits shown in the various figures, there is a differentiallywound relay included in the answering sleeve strand. Those differential relays 37 of Fig. 2, 57 of Fig. 3, 67 of Fig. l, 77 of Fig. 5, and 87 of Fig. 6, function in very much the same manner, practically the only ditl'erence between these relays being in the number of contacts controlled thereby. The dili 'erential relay 17 of Fig. 1 also functions like those of the figures mentioned in the preceding sentence, but it has the additional function of controlling the connection of the operators telephone set to the cord circuit, and is so constructed as to attract only a portion of its armatures when it is only partially energized, as when the circuits including both windings are completed, the resistance 31 being in circuit with the lower winding thereof. The difierential relay 97 of Fig. 7 differs from those differential relays of the other figures principally in that the two windings thereof are connected in series opposing in the same circuit rather than in multiple circuits.

From the descriptions of the various circuits shown in the drawings, it will be evident that applicant has provided a system in which a very small amount of apparatus is required to cause the intermittent operation of a signaling device to furnish a recall si nal to the operator.

What is claimed is:

1. A telephone exchange system comprising telephone lines terminating at a central office, a link circuit thereat connected with one of the lines, a signaling device in the link circuit, a supervisory relay for the link circuit, an interrupter, and a difierentiallywound relay in the link circuit when energized cooperating with the supervisory relay when energized to connect the interrupter in circuit with the signaling device.

2. A telephone exchange system comprising telephone lines terminating at a central otiice, a link circuit thereat, a listening key, a supervisory relay, a differential relay operating upon connection of the link circuit with one of the lines and releasing upon the actuation of the listening key, a signaling device, and interrupter mechanism brought into circuit therewith upon the subsequent operation of the difierential relay followed by the operation oi the supervisory clay.

3. A telephone exchange system comprising telephone lines, a link circuit, a relay having an operating winding and a new tralizing winding, a circuit including the operating winding of the relay completed.

upon connection or the link circuit with one of the telephone lines, a supervisory relay for the link circuit, an operators telephone set, a circuit including the neutralizing winding completed when the operators telephone set is connected withthe link circuit, a signaling device, interrupter mechanism, and a circuit for the signaling device and the interrupter mechanism including contacts of both relays. v

4. A telephone exchange system comprising telephone lines, a link circuit, a relay having an operating winding and a neutralizing winding, a circuit including the operating winding of the relay completed upon connection of the link circuit with one of the telephone lines, a supervisory relay for the link circuit, an operators telephone set, a circuit including the neutralizing winding completed when the operators telephone set is connected with the link circuit, a sigraling device, interrupter mechanism, and a circuit including the signaling device and he interrupter mechanism established upon the subsequent operation of the first mentioned relay followed by the operation of the supervisory relay.

5. A. telephone exchange system comprising telephone lines, a link circuit, a primary relay having an operating and a neutralizing winding, a circuit including the operating winding completed upon connection of the link circuit with one of the lines, a supervisory relay for the link circuit, a listening key, a circuit including the neutralizing winding completed upon the actuation of the listening key, a signaling device, interrupter mechanism, and a circuit for the signaling device and the interrupter mechanism including contacts of both the primary and the supervisory relays.

6. A telephone exchange system comprising telephone lines, a link circuit, a primary relay having an operating and a neutralizing winding, a'circuit including the operating winding completed upon connection of the link circuit with one of the lines, a supervisory relay for the link circuit, a listening key, a circuit including the neutralizing winding completed upon the actuation of the listening key, a signaling device, interrupter mechanism, and a circuit for the signaling device and the interrupter mechanism established upon the subsequent operation of the primary relay followed by the operation of the supervisory relay.

7. A telephone exchange system comprising telephone lines, a link circuit, a primary relay having an operating and a neutralizing winding, a circuit including the operating winding completed upon connection of the link circuit with one of the lines, a supervisory relay for the link circuit, a listening key, a circuit including the neutralizing winding completed upon the actuation of the listening key, a signaling device and interrupter mechanism, and a circuit therefor open at two points, closed at one point upon the operation of the primary relay when the supervisory relay is deelnergized and at the other point by the subsequent energization of the supervisory relay.

8. A telephone exchange system comprising telephone lines, a link circuit, a primary relay having an operating and a neutralizing winding, a circuit including the operating winding completed upon connection of the link circuit with one of the lines, a supervisory relay for the link circuit, a listening key, a circuit including the neutralizing winding completed upon the actuation of the listening key, a signaling device and interrupter mechanism, and a circuit therefor Copies of this patent may be obtained for five cents each, by addressing the Commissioner of Patents, Washington, D. O. 

